A head-mounted display (HMD) system is a type of wearable device with increasing popularity within the consumer electronics industry. HMDs, along with similar devices such as helmet-mounted displays, smart glasses, and virtual reality headsets, allow users to wear a display device such that the hardware remains fixed to their heads regardless of the user's movement.
When combined with environmental sensors such as cameras, accelerometers, gyroscopes, compasses, and light meters, HMDs can provide users with experiences in virtual reality and augmented reality. Virtual reality (VR) allows a user to be completely submerged into a virtual world where everything the user sees comes from the display device. Devices that provide augmented reality (AR) allow users to optically see the environment, and images generated by the display device are added to the scene and may blend in with the environment. Accordingly, traditional VR and AR technology involve a display that is mounted in front of the user's head with a lens configuration that determines the virtual image position and field.
A basic layout of typical commercial VR or AR systems (both involving and not involving use of a smartphone for a display) includes a display device and a lens structure that images the display light into the far field to enable comfortable viewing. To ensure sufficient magnification, with wide field of view and to have a virtual image at a far enough distance from the eye, the size of this arrangement is restricted. In addition, the display is a relatively far distance from the eyes, meaning that the device must be strapped to the head to not fall off. Furthermore, the weight of the device is far forward when worn, meaning that long term viewing could become tiresome on the face and neck due to the torque generated about the head by the weight of the device. Lens elements used in such systems may be configured as a normal curved surface lens of known type, or a structured Fresnel lens with angled features of known type, or other known lens arrangements involving one or more lenses.
One of the primary elements of HMDs is the display module mounted onto the head. However, since the unaided human eye cannot accommodate (that is, change optical power to provide a focused image) for images closer than a certain distance from the eye, eyepiece lenses are required to re-image the display module such that the display image appears to be at a comfortable viewing distance from the user. Such optical configuration requires substantial space between the eyepiece and the display module. Furthermore, complex lenses are needed if the HMD needs to display images with high quality and a wide field of view (FOV). The result of these requirements in conventional systems is a heavy and bulky headset that is uncomfortable to wear for any length of time, and the size is limited by basic optics to achieve the correct magnification and the virtual image distance.
Due to the bulkiness of conventional configurations, portability has become an issue in that conventional HMD systems take up substantial space for storage and transport. In one type of HMD systems, image panels are permanently installed into the HMD system with appropriate magnifying optics. This type of HMD arrangement includes a display device and lens components that display an image into the far field to enable comfortable viewing. The lens and display are both mounted within some form of fixed housing. A fixed housing in this regard is a structure that maintains the optical components' relative positions and orientations from which the optical components cannot be removed. In this respect, the display is considered one of the optical components. Many such systems are not foldable and have a fixed form factor. In alternative arrangements, attempts have been made to reduce the form factor, which involve keeping the optical components fixed and providing folding parts of the HMD housing, such as with respect to head-straps and/or side-arms. As the size of display and lens-display distance determines the form factor in simple magnifying HMDs, there is little decrease in the form factor by using these methods of providing foldable housing structures while maintaining the optical components fixed.
In another type of HMD system, a frame or housing is provided to incorporate or “drop-in” a smartphone. With drop-in smartphone type HMD systems, it is typically expected that the user will remove the smartphone after use with the HMD system. This removes the lens-display distance constraint and allows the HMD frame or housing to be foldable or otherwise re-configurable in absence of the smartphone. A reconfigurable housing in this regard is a structure that maintains the optical components' relative positions and orientations from which the optical components can be inserted and removed. In typical commercial systems, the display is implemented in the form of a smartphone that can easily be removed from the housing with a snap-fit or similar operation. When one or more of the optical components are removed from the housing (e.g., by removal of a smartphone), the housing can then be reconfigured in some way (e.g. folding, bending) so that it can be conveniently stored and transported. The benefit of portability, however, is generally offset by the lower image quality of drop-in smartphone type HMD systems.
There have been other attempts to reduce the HMD form factor using multiple small lenses with overlapping images that modify the magnification required. For example, the Applicant's commonly owned Application GB 1621621.0, filed Dec. 19, 2016, describes an alternative image overlap system with two displays and a folded W-shaped mirror arrangement. The field of view in this case is defined by the maximum aperture and path length. Although such a system reduces the overall form factor during use as compared to prior conventional configurations, in this initial design the form factor is fixed and therefore the reduction of the form factor during periods of non-use, such as for storage and transport, still have been limited by the optical requirements of usage.